This invention relates to a system for measuring the values of living body tissue characteristics n and .beta. which uses a reflected ultrasonic wave, where .beta. is a proportionality constant called the attenuation slope describing the tissue attenuation constant being proportional to .beta.f (f is the frequency of the ultrasonic wave) and where n is a power exponent constant called the reflection power exponent describing the tissue reflection coefficient being proportional to the frequency of the ultrasonic wave raised to the power n, and more particularly, to a method for separately obtaining n and .beta. from the shape of the power spectrum of the reflected ultrasonic wave.
Two previous methods, different from the method used by the present invention, for obtaining the proportionality constant .beta. (that is, the attenuation slope) have been proposed by Dr. Miwa et al. in the following applications:
1. An energy ratio method where the value of .beta. is obtained from the energy ratio of two frequency bands. This method is disclosed in Japanese patent application No. 55-74680, filed June 3, 1980 having corresponding U.S. patent application Ser. No. 269,861, filed June 3, 1981, now U.S. Pat. No. 4,414,850, the contents of which are hereby incorporated by reference.
2. A center frequency shift method where a value of .beta. is obtained from the shift in the center frequency of a power spectrum of a Gaussian distribution ultrasonic pulse as it passes through tissue. This method is disclosed in Japanese patent application No. 56-65536, filed Apr. 30, 1981 having corresponding U.S. patent application Ser. No. 372,547, now U.S. Pat. No. 4,452,082, filed Apr. 28, 1982 the contents of which are hereby incorporated by reference.
The above-mentioned methods are based on the assumption that the reflected coefficient does not depend on frequency. Actual experimental evidence proves that the reflected coefficient is proportional to the frequency raised to the power n, where n is a constant within the range from 0 to 6 depending on the specific tissue type and indicates a particular characteristic of the tissue. Thus, because of the above-mentioned assumptions related to the reflection coefficient, the above-mentioned methods result in considerable error.
Another method different from the method of the present invention for obtaining the values of .beta. and n by analyzing reflected ultrasonic waves where the values .beta. and n are combined has been proposed by Dr. Miwa et al. in a method which utilizes three frequencies. This method is an extension of the energy ratio method mentioned above and is disclosed in Japanese patent application 57-57573 (also EP No. 91768, published Oct. 19, 1983), filed Apr. 7, 1982 the contents of which are hereby incorporated by reference. This method is effective for analyzing living body tissue but has a drawback in that errors due to local fluctuation of the spectrum can be introduced. The local fluctuation of the spectrum results from the mutual interference of overlapped pulse echoes, which is known as scalloping and is due to the narrow frequency bands used in the three frequency method. This method has a further disadvantage in that a large number of calculations are necessary because of the various combinations of the three frequencies within the effective frequency band that must be performed to provide an ensemble of n and .beta. values for statistical processing. The present invention utilizes the entire pulse spectrum and is called a spectrum shape method.